This invention pertains generally to radio frequency oscillators, and, more particularly, to a low noise, frequency-agile microwave oscillator.
Modern day air defense radars require the use of microwave sources with extremely low noise to counter the threat of stealth aircraft or to achieve better moving target indicator (MTI) performance. Further, to operate satisfactorily in the presence of electronic countermeasure (ECM), or to control a multiplicity of surface-to-air missiles, such radars must be frequency-agile.
In the past, very low noise and frequency agility could not be attained contemporaneously. Thus, known microwave sources achieving very low noise operation do so through the use of a cavity discriminator, while known microwave sources achieving frequency agility do so through the use of an interferometer-controlled feedback loop to degenerate phase noise. Therefore, with known design techniques, it is necessary in each application to make a "trade-off" between noise and frequency agility.
There are several secondary problems associated with the use of a cavity that complicate its use as a noise degeneration mechanism for a microwave source. The first such problem is the bandwidth of the cavity: If the bandwidth of the cavity is too wide, then sensitivity is reduced, but if the bandwidth is too narrow, then stability is affected. The bandwidth of any cavity is determined by the "Q" of the cavity, a parameter that is a function of geometry, surface finish, and other mechanical considerations, so fabrication of a satisfactory cavity for the purpose is difficult. Further, there is also a problem of microphonics that may be encountered in the use of any cavity.